1. Field of the Invention
The present invention relates to projection optical systems projecting light from an original image onto a projection surface, and to image projection apparatuses having such a projection optical system.
2. Description of Related Art
As projection optical systems used for projectors (image projection apparatuses), projection optical systems have been proposed, which can project images obliquely onto a screen in order to perform image projection without obstructing the visual field of the viewers. Moreover, since so-called trapezoidal distortion occurs in the projection image when projecting images obliquely onto the screen, there have been proposals regarding the correction of this trapezoidal distortion.
The projection optical system proposed in Japanese Patent Application Laid-Open No. H9(1997)-304733 (referred to in the following as Document 1) is an application of a coaxial rotationally symmetric f-θ lens, and rotates the image surface in a state in which there is no trapezoidal distortion of the image due to rotationally decentering a lens unit around the aperture stop. Moreover, the projection optical system proposed in Japanese Patent Application Laid-Open No. H9(1997)-282451 (referred to in the following as Document 2) corrects trapezoidal distortion with a fixed projection angle by using a decentered aspherical surface.
On the other hand, regarding off-axial optical systems, a design method and method of calculating paraxial amounts such as the focal length for off-axial optical systems have been disclosed in Japanese Patent Application Laid-Open No. H9(1997)-005650 (referred to in the following as Document 3), and design examples of off-axial optical systems are shown in Japanese Patent Application Laid-Open No. H8 (1996)-292371, Japanese Patent Application Laid-Open No. H8(1996)-292372, and Japanese Patent Application Laid-Open No. H9(1997)-222561, and it has become clear that it is possible to construct optical systems with sufficient correction of aberrations by introducing the concept of reference axis and using asymmetric aspherical surfaces as the constituent surfaces.
Such off-axial optical systems are defined as optical systems which, when the path of the light ray passing through the image center and the pupil center is defined as the reference axis, include curved surfaces whose surface normal at the intersection of the constituent surface with the reference axis is not on the reference axis (off-axial curved surfaces), and in which the reference axis takes on a bent shape.
In such off-axial systems, the constituent surfaces are ordinarily non-coaxial, and there is no vignetting at the reflective surfaces, so that it is easy to construct an optical system using reflective surfaces. Moreover, a compact optical system with a broad field angle can be configured by forming an intermediate surface within the optical system. Furthermore, the light path can be guided relatively freely while using a front-aperture optical system, so that a compact optical system can be configured.
Utilizing these characteristic features, Japanese Patent Application Laid-Open No. 2001-255462 (corresponding to U.S. Pat. No. 6,626,541 and EP 1139145 (A2)) and No. 2000-089227 propose a projection optical system in which trapezoidal distortion at a fixed projection angle is corrected using curved rotationally asymmetric reflective surfaces.
The projection optical system proposed in Document 1 uses a combination of coaxial rotationally symmetric f-θ optical systems, but its field angle is narrow, so that it is difficult to apply to ordinary projectors or the like. Moreover, with ordinary lenses, the light amount is reduced as the field angle from the optical axis becomes large, so that there are greater differences in the brightness on the image surface the broader the field angle of the used lens system is. For this reason, the projection optical system in Document 1 is not suited for projectors that necessitate a broad field angle and a bright image surface.
The projection optical system proposed in Document 2 corrects the trapezoidal distortion with decentered aspherical surfaces. However, since it is a projection optical system that can correct the trapezoidal distortion only at a fixed projection angle, the degree of freedom for the positions at which the projection apparatus can be set up is compromised. Moreover, regarding the fact that the shift amount of the image surface is small and that it is not telecentric with respect to the liquid crystal panel, it is not suited for projectors.
The projection optical systems in Documents 3 and 4 accomplish a correction of the trapezoidal distortion over a broad field angle using curved rotationally asymmetric reflective surfaces. However, they are projection optical systems with which the trapezoidal distortion can be corrected only at a fixed projection angle, so here, too, the degree of freedom for the positions at which the apparatus can be set up is compromised.